A. High Porosity Vascular Prostheses (HPVP)
The ideal specifications for vascular prostheses have been elaborated by Wesolowski and include: (1) the absence of toxicity, allergenc potential, or other overtly adverse chemical reaction, (2) durability upon prolonged implantation, (3) the biological healing porosity should be on the order of 10,000 ml of water/min. per sq cm of fabric at a pressure head of 120 mm Hg, (4) the material should have a low implant porosity to enable the administration of heparin or other anticoagulant: less than 50 cc/min/cm.sup.2 at a pressure head of 120 mm Hg, (5) there should be desirable handling properties which would facilitate implantation, for example conformability for easy anastomosis, linear elasticity (allowed by crimping), pliability and good twist characteristics. Wesolowski et. al. (1968) Ann. N. Y. Acad. Sci. 146 pp. 325-344. Commercially available high porosity prostheses meet some of the above specifications but still face the problem of prohibitive leakage of blood at the time of implantation.
Porosity refers to the proportion of void space (pores) within the boundaries of a solid material, compared to its total volume Guidoin et. al. (1987) J. Biomed Materials Research 21 pp. 65-87. The importance of porosity in determining the long term performance has been emphasized Edwards, W. S. (1957) Surg. Forum 8 p. 446. This study showed the necessity for pores that will permit fibroblasts to grow into the lumen of the graft, organize the fibrin, and heal the graft in place. The porosity is believed to give a rough prediction of the capacity of the new graft to anchor newly formed surrounding tissue after implantation, i.e. the greater the porosity the better the healing Guidoin et. al. Prostheses that are woven too tightly may fail to allow ingrowth of host tissue. Wesolowski et. al. (1961) Surgery 50 p. 91.
The methods and materials that have been used to prevent transprosthetic bleeding during surgery include: preclotting with autologous blood, fresh frozen plasma, platelets, or albumin, or coating the graft with fibrin glue at the time of operation. These methods have many potential drawbacks, such as persistent bleeding, incomplete or less than optimal sealing, thromboembolism and bacterial contamination. Jonas et. al. (1986) Ann. Thorac. Surg. 41 pp. 657-663; Gloviczki et. al. (1984) J. Thorac. Cardiovasc. Surg. 88 p. 253; Wooster et. al. (1985) Can. J. Surg. 28 pp. 407-409. These methods include the risk of blood transmitted diseases if homologous blood or its derivatives are used for preclotting. In addition, many of these materials are costly.
Gelatin has been used to impregnate vascular prostheses and the gelatin appeared to be rapidly reabsorbed with minimal influence on healing. However, these grafts had problems; they were brittle and their permeability easily increased after manipulation. Bascom, J. U. (1961) Surgery 50 pp. 504-512.
The use of strongly cross-linked collagen prostheses has been found to give an unacceptable increase of permeability after manipulation, and is associated with a delayed reabsorption of collagen and with prevention of capillary growth. Jonas et. al. The use of collagen coated prostheses has also been associated with problems of suspected immunological reaction, which has prevented the wide acceptance of these prostheses. Balzer et. al. (1988) Thorac. Cardiovasc. Surg. 36 pp. 351-355.